Mode partition noise screening apparatus

ABSTRACT

Accurate mode partition data from a laser are collected simply and rapidly in accordance with the principles of the invention wherein the laser output is directed to a filter which separates a central longitudinal mode from the side modes. The filter operates to present all side modes in a predetermined wavelength range simultaneously and continuously at the output of the filter. By subsequently comparing the intensity or power in the side modes delivered to the filter output with a predetermined threshold, it is possible to determine the frequency of occurrence and magnitude of mode partition events for side modes in the predetermined wavelength range. In one exemplary embodiment, the filter is realized by the combination of a Fabry-Perot etalon, whose longitudinal axis is angled with respect to the longitudinal axis (propagation axis) of the laser output beam directed onto the filter, together with an external reflector which is parallel to the Fabry-Perot etalon to permit multi-passing of the beam between the Fabry-Perot etalon and the reflector. The beam is reflected by the Fabry-Perot etalon so that after two or more passes within the filter only the side modes are output by the filter. In this arrangement, the filter acts as a notch filter having a wide passband and having an output for the side modes which is separate from an output for the main mode.

This is a continuation of application Ser. No. 07/585763 filed Sept. 19,1990, U.S. Pat. No. 5,056,101

TECHNICAL FIELD

This invention relates to laser devices and, more particularly, toapparatus for characterizing modal behavior of the laser devices.

BACKGROUND OF THE INVENTION

Semiconductor lasers tend to have output spectra consisting of multiplelongitudinal modes. Depending on the relative strengths of these modes,the lasers are classified as multi-mode or single mode. Single modelasers are nominally single mode because their output comprises adominant main mode and several small, yet measurable, modes atwavelengths near the wavelength of the main mode. While the averagepower in each side mode is usually quite small, the side modes aregenerally present. Moreover, the power of each individual longitudinalmode fluctuates from a zero power level to significant percentage of afull output power level at any time.

When a single mode laser is directly modulated to carry information in alightwave communication system, the spectrum of each modulated digitcomprises light output from the laser at the wavelengths correspondingto the main mode and to those side modes which were present during theformation of the particular information digit. After the modulated digittraverses a length of dispersive optical fiber, the differentwavelengths of the longitudinal modes cause the modes comprising a digitto lose temporal correlation which may result in an error at thereceiver. The error phenomenon caused by mode partitioning of thetransmitting laser is especially exhibited in ASK and OOK systems. Forsystems employing high speed modulation and with transmission over longdistances, mode partitioning degrades error rate performance and oftenintroduces so-called "error rate floor" characteristics in the errorrate versus average optical power system specifications. While lasersare being constantly refined to eliminate or just ameliorate the effectsof mode partitioning, it is necessary to determine the propensity of thelaser toward mode partitioning and degree of mode partitioning for eachlaser prior to installation in a system.

Various techniques have been demonstrated for capturing modepartitioning data from a modulated laser so that the performance of thelaser may be characterized. The variety of techniques cover eye diagrammapping of the error rate, side mode suppression ratio analysis,inducing error by varying total dispersion, analyzing kinks inlight-output versus current (L/I) curves, and mode sampling byspectrometer. In general, the latter technique is most widely accepted.

In eye diagram mapping, the receiver decision point is varied inamplitude and phase to collect an ensemble of different error rates. Thelaser under test transmits its signal over a full transmission system tothe lightwave receiver. By plotting constant error rate contours asclosed loops, it is possible to understand quantitatively the error ratedistribution in the eye pattern. See, for example, J. of LightwaveTech., Vol. 6, No. 5, pp. 678-685 (1988). It is said that this techniqueand the eye pattern supply information about degraded performanceresulting from an error rate floor and low probability phenomena whichcharacterize laser performance. Even for high data rate systems, thistest procedure requires long periods of time for data collection toobtain a statistically significant sampling of laser performance.Clearly, this technique is time consuming and provides onlycircumstantial evidence of mode partitioning in characterizing the laserat the remote end of the communication system. The latter detriment isso because the eye pattern data is assembled by collecting only errorrate data from the receiver and then by stating without furthersupporting data that error performance of the laser was caused by modepartitioning events rather than by collecting mode partition event datatogether with the error rate data from the laser under test and thentrying to correlate that partition event data with the error rate data.

Side mode suppression ratio analysis and analysis of kinks in the L/Icurve for the laser have been found to provide a rough approximation ofmode partitioning activity. For a description of the former technique,see J. of Lightwave Tech., Vol. 6, No. 5, pp. 636-642 (1988). Whilethese techniques are known to be useful, their utility is realisticallylimited to obtaining merely indications of mode partitioning withoutqualitative support.

For the mode distribution sampling by spectrometer technique, a numberof references have shown similar experimental configurations. See, forexample, IEEE Trans. on Commun., Vol. COM-28, No. 2, pp. 238-243 (1980);J. of Lightwave Tech. Vol. LT-2, No. 1, pp. 44-48 (1984); and J. ofLightwave Tech. Vol. LT-3, No. 3, pp. 706-712 (1985). In general, thesereferences show the laser output focused on a monochromator which isresolved to pass one longitudinal mode of the laser at any given time.High speed sampling is then used to record a historical record of thefluctuations of the selected mode being passed by the monochromator.Since many other longitudinal modes are rejected by the monochromatorand after a statistically significant amount of data has been collectedfor the one mode being analyzed, it becomes important for completenessof this technique to adjust the monochromator to pass anotherlongitudinal mode of the laser. Unfortunately, this technique isextremely time consuming, while producing mode partition data which isinaccurate because the data about partition events are collectedsequentially from one longitudinal mode to the next. Infrequent, yetvery important, partition events in other modes are missed when themonochromator is not resolved on the modes which are experiencing apartition event. As a result, statistical techniques are used duringlater data analysis to try to fill in missing data about partitionevents. It is also noted that mode sampling by spectrometer techniquesare both polarization sensitive (dependent) and inherently lossy.Polarization sensitivity introduces additional complexity in the testprocedure by requiring that the laser and the test equipment be alignedto optimize mode coupling and power transfer. Loss, on the other hand,is problematic when one considers the usual low power emitted in themodes to either side of the main (central) longitudinal mode of thelaser.

SUMMARY OF THE INVENTION

Accurate mode partition data from a laser are collected simply andrapidly in accordance with the principles of the invention wherein thelaser output is directed to a filter which separates a centrallongitudinal mode from the side modes. The filter operates to presentall side modes in a predetermined wavelength range simultaneously andcontinuously at the output of the filter. By subsequently comparing theintensity or power in the side modes delivered to the filter output witha predetermined threshold, it is possible to determine the frequency ofoccurrence and magnitude of mode partition events for side modes in thepredetermined wavelength range.

Polarization independent operation is achieved in all the inventiveembodiments. Moreover, because all side modes are collectedsimultaneously and continuously, all mode partition events which occurin the predetermined wavelength range of the filter during datacollection are measurable. Therefore, the mode partition data iscollected with an accuracy heretofore unachievable.

In one exemplary embodiment, the filter is realized by the combinationof a Fabry-Perot etalon, whose longitudinal axis is angled with respectto the longitudinal axis (propagation axis) of the laser output beamdirected onto the filter, together with an external reflector which isparallel to the Fabry-Perot etalon to permit multi-passing of the beambetween the Fabry-Perot etalon and the reflector. The beam is reflectedby the Fabry-Perot etalon so that after two or more passes within thefilter only the side modes are output by the filter. In thisarrangement, the filter acts as a notch filter having a wide passbandand having an output for the side modes which is separate from an outputfor the main mode.

Additional features of the invention include having the filter outputthe main longitudinal mode of the laser output, tunability of the filternotch wavelength, controllability of the filter notch FWHM, andemploying the intensity or power of the main longitudinal mode or avalue related thereto as the comparison threshold. Exemplary elementsembodying the features above are disclosed in the following description.Also, alternative filter arrangements are presented.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the invention may be obtained byreading the following description of a specific illustrative embodimentof the invention in conjunction with the appended drawing in which:

FIG. 1 shows a block diagram of apparatus for measuring dynamic modepartitioning in accordance with the principles of the invention;

FIG. 2 shows a plot of calculated intensity reflectivity versuswavelength for a Fabry-Perot interferometer employed in the apparatusshown in FIG. 1;

FIG. 3 shows plots of the laser spectrum before and after filtering bythe optical notch filter shown in FIG. 1;

FIG. 4 shows a plot of the transfer function for the optical notchfilter shown in FIG. 1 and characterized in FIG. 3; and

FIG. 5 shows distributions of mode partition pulse heights for anexemplary single mode laser under two different bias conditions asmeasured using the apparatus of FIG. 1.

The various exemplary embodiments of the invention shown in the FIGS.have not been drawn to scale and are merely for purposes of illustrationand not limitation. Exemplary dimensions of the devices shown in theFIGS. are stated hereinbelow.

DETAILED DESCRIPTION

Mode partition detection is achieved by utilizing an optical filterarrangement which provides an accurate, broadband characterization ofside longitudinal mode activity for a laser whose light beam is directedonto the filter. The optical filter arrangement in different embodimentsis configured as a multi-port device in which the laser light beam isdirected onto one port and in which, after separation in the filter, theside longitudinal modes are delivered to a second port while the main orcentral longitudinal mode is delivered to an optional third port. Ineffect, the optical filter notches out the light in the main mode andleaves only the light in the laser side modes. The transfer function ofthe filter response bears resemblance to a notch filter characteristic.As a result, mode partitioning measurements are made by detecting a weakpartitioning pulse effectively without the main mode being present sothat presence of a pulse indicates the existence of a mode partitionevent while the magnitude of the pulse indicates the qualitative degreeof partitioning.

An exemplary embodiment of the arrangement is shown in FIG. 1. Soliddirected lines represent electrical signal paths whereas dashed linesrepresent optical signal paths. As shown in FIG. 1, laser 2 is biased ator near a lasing threshold and electrically modulated such as OOK or ASKby pattern generator 1. As such, the lower intensity (OFF) state of thelaser occurs at or near the lasing threshold. The output light beam fromthe laser is said to be intensity modulated and is generally coupledfrom the laser into a short pigtail (not shown) of single mode fiberwhich is lensed at the end interfacing with the laser. It should benoted that laser 2 exhibits nominally single longitudinal mode behavior.Such lasers include distributed feedback (DFB) lasers, distributed Braggreflector (DBR) lasers, coupled cavity and cleaved-coupled cavity (C³)lasers, and the like. A light beam from laser 2 is supplied to isolator3 which is a standard optical isolator designed to prevent returnreflections from splices, mirrors or interconnections from entering thelaser cavity and interfering with normal operation. Coupling lens 5 isshown collimating the laser output light beam into filter arrangement 6.

Filter arrangement 6 includes a Fabry-Perot etalon formed by reflectors7 and 8 in combination with external reflector 9. The Fabry-Perot etalonis tilted slightly by an angle θ, usually less than 5°, to have itslongitudinal axis slightly angled from the propagation (longitudinal)axis of the light beam. The longitudinal axis of the Fabry-Perot etalonis the axis normal to reflectors 7 and 8. The titled relationshipbetween the Fabry-Perot etalon and the light beam permits the reflectedwavelengths from the Fabry-Perot etalon to exit from reflector 7 along apath which does not interfere with the incoming light beam from laser 2.It should be noted that the tilt angle θ is selected to avoid reducingfinesse of the Fabry-Perot etalon. External reflector 9 returns thelight exiting the Fabry-Perot etalon through relector 7 to theFabry-Perot etalon; external reflector 9 is parallel to reflectors 7 and8 of the Fabry-Perot etalon to permit the light beam emerging fromreflector 7 to effectively "walk off" the surfaces of the Fabry-Perotetalon and reflector 9. External reflector 9 is generally very highreflectivity and is separated from the Fabry-Perot etalon by a distanceL which, together with the tilt θ of the Fabry-Perot etalon and thediameter α of the radiated beam causes the light at wavelengthsreflected by the Fabry-Perot etalon to be returned to the Fabry-Perotetalon for additional passes. A minimum separation distance L is givenas approximately a/θ, where θ is small and expressed in radians. Asshown in FIG. 1, external reflector 9 causes the light beam to undergothree passes in the Fabry-Perot etalon before finally emerging on outputcoupling lens 15. In order for filter arrangement 6 to provide a notchtransfer characteristic which substantially removes the mainlongitudinal mode, the Fabry-Perot etalon is tuned to the wavelength ofthe main longitudinal mode of laser 2. It is understood that the sidelongitudinal modes representing the mode partitioning events arecontained in the light beam emerging from the reflection side of theFabry-Perot etalon through reflector 7.

For the arrangement shown in FIG. 1, transverse dimensions of reflector9 are selected to permit the desired multi-passing effect while alsopermitting the laser output beam and the final output beam from theFabry-Perot etalon to pass therearound without interference. In general,minimum transverse dimensions of reflector 9 are on the order of twobeam diameters.

As stated above, the main longitudinal mode to which the Fabry-Perotetalon is tuned is output from the transmission side of the Fabry-Perotetalon through reflector 8. This light beam is detected by detector 10which may include a PIN photodetector. Detector 10 supplies its outputto control element 11 which uses the low frequency components of thesupplied signal to lock the Fabry-Perot etalon to the wavelength of themain longitudinal mode of laser 2.

Once the side longitudinal modes have been separated from the mainlongitudinal mode by the filter arrangement 6, the side modes aresuitable for data collection and processing. A standard coupler 12guides the side mode light beam to optical receiver 13. Coupler 12 mayhave additional output ports and, therefore, may permit connection ofother output devices such as an optical spectrum analyzer for additionaldata collection functions. Optical receiver 13 is a standard square lawdevice such as a high speed photodetector circuit having sufficientbandwidth, at least several GHz, to capture the high speed modepartition events. AC-coupling of the receiver is generally employed toignore DC components and to view more clearly the fast mode partitioningevents. The output from optical receiver 13 is supplied to comparator14. A threshold level is supplied to comparator 14 to determine theappropriate level for accepting mode partition events. When the signalfrom receiver 13 exceeds the threshold at comparator 14, the comparatoroutputs a signal which indicates the occurrence of a mode partitionevent and, if desired, the magnitude of the mode partition event. In anexample from experimental practice, a real-time oscilloscope (Tektronix7104) with 1 GHz bandwidth was employed as high speed comparator 14wherein the threshold was provided to the internally triggeredoscilloscope input through a standard bias tee with the signal fromreceiver 13. A counter coupled to the gate output of the oscilloscoperecorded the data relating to the number of partition events whichexceeded the threshold in a particular time period.

Isolator 3 and coupler 12 have been displayed as dashed boxes toindicate that their use is optional. It should also be noted that inputcoupling lens 5 and output coupling lens 15 may be eliminated when theinput and output beams for filter arrangement 6 are collimated.

In an example from experimental practice, a plano Fabry-Perot etalon wasused incorporating a Burleigh Model RC-110 with 2-inch diameter mirrorshaving NR-1 coatings as reflectors 7 and 8. The transmission peak of theetalon was positioned at the wavelength corresponding to the mainlongitudinal mode of the laser and the reflected light was monitoredthrough lens 15 to determined mode partitioning. As shown in FIG. 2, thewavelength dependence of the signal reflected by the Fabry-Perot etalonexhibits an optical notch filter characteristic in which the mainlongitudinal mode is rejected, that is, separated from the side modes,and the side modes are passed. This rejection is shown as notch 22 ofcurve 21. Units for the ordinate are measured as the ratio of thereflected signal to the input signal powers. In the example, theFabry-Perot etalon had a finesse of approximately 80 and a plateseparation of 15 μm. These parameters give rise to a free spectral rangefor the Fabry-Perot etalon of approximately 80 nm when the full width athalf minimum is approximately 1 nm. It should be noted that, for thisexample, laser 2 was an 1.7 Gbps ASK modulated DFB semiconductor laserhaving an operating wavelength of approximately 1.5 μm. The operatingwavelength is the location of the main longitudinal mode of the laser.

For the arrangement shown in FIG. 1, the number of passes undergone bythe laser output light beam in filter 6 determines the degree ofrejection experienced by the main mode. We have determined that, for asingle pass, the main longitudinal mode was rejected by approximately 14dB. For two passes, this rejection capability was increased toapproximately 30 dB. Ultimately, by adding a third pass, the Fabry-Perotetalon exhibited a main mode rejection capability greater than 45 dB. Itwill be understood by those skilled in the art that basic techniquesknown in the field of optics are utilized to vary the characteristics ofthe filter with respect to tuning to the main mode wavelength andvarying the amount of main mode rejection. These techniques include, butare not limited to, adjusting the tilt of the Fabry-Perot etalonrelative to the laser output beam, adjusting the tilt of the externalreflector relative to the longitudinal axis of the Fabry-Perot etalon,varying the distance between the external reflector and the Fabry-Perotetalon, and changing the reflector spacing of the Fabry-Perot etalon.

For the exemplary optical notch filter described above, filtering of thelaser output beam is shown in FIG. 3. Curve 31 shows the spectrum of theoutput light beam from the laser under ASK modulation as describedabove. Curve 32 shows the light beam output from the filter after thethree passes. As seen from a comparison of curves 31 and 32, most of thelight corresponding to the main longitudinal mode has been rejected by(i.e., separated from the side longitudinal modes) filter 6. It shouldbe noted that some light corresponding to the main mode signal occurs incurve 32 because, at every OFF-ON transition of the ASK modulated laseroutput beam, the main mode is caused to chirp out of the filterbandwidth. Chirping is a well known frequency modulation process. Bytaking a ratio of curves 31 and 32, it is possible to calculate thetransfer function of filter 6. This transfer function is shown as curve41 in FIG. 4. From this transfer function, it is apparent that, for awavelength difference greater than 1.5 nm away from the notch, only 0.3dB of the light in the side longitudinal modes is lost. As such, theexemplary filter described above is an excellent tool for measuring modepartitioning.

In operation, the mode partitioning signal takes the form of pulseswhich occur infrequently and at random ON transitions within thepseudo-random sequence from the pattern generator. As observed, thefrequency of the pulses appears to be less than approximately 10⁴ sec⁻¹.Pulse widths are limited by the bandwidth of the oscilloscope used incomparator 14, while pulse heights are distributed over a widecontinuum. In the example above, pulse widths are limited to a durationof approximately 1 ns. With respect to pulse heights, the distributionis such that smaller pulses occur more frequently than large pulses.Mode partitioning is measured using the oscilloscope trigger circuit asa high speed comparator to output a pulse whenever the input signalexceeds an applied threshold. In order to obtain a distribution of modepartition pulse heights, the threshold level is varied over the range ofexpected pulse heights. For each threshold level, output pulses from thecomparator are counted for a sufficient period of time to obtain anaverage partitioning rate for pulses having a height greater than orequal to the predetermined threshold level. In our experimentalpractice, the period of time necessary to achieve averaging was between1 and 100 seconds. The cumulative distributions of mode partition pulseheights for one DFB laser are shown in FIG. 5. Distribution 51 (opensquares) represents the distribution when the laser is biased at 1.0I_(th) ; distribution 52 (filled circles) represents the distributionwhen the laser is biased at 0.6 I_(th). As shown in FIG. 5, the observedpartitioning rate decreases as a substantially exponential function ofthe pulse height. The measured partitioning distribution can beappropriately convolved with the receiver noise characteristic toestimate the maximum error floor of the laser under test.

Filter 6, as described herein, provides a notch characteristic which issufficiently broad to remove the main longitudinal mode of mostsemiconductor lasers with chirp. It also provides a notch characteristicwhich is sufficiently deep to expose weak activity in the sidelongitudinal modes. The filter shown above provides tunability byvarying the reflector spacing at least over the wavelength range from1.25 μm to 1.65 μm. The particular filter shown in FIG. 1 passes asignal which has a magnitude related to the cumulative strength of allpartitioning modes. Therefore, the filter arrangement provides a measureof mode partitioning in the worst case. Additional benefits of thefilter arrangement shown in FIG. 1 are polarization independence, lowloss, and realization using commercially available optics and electroniccomponents.

Tunability of the filter is a desirable feature because most lasers varywith respect to the absolute wavelength of the main longitudinal mode.This is true even for lasers which are grown on the same semiconductorwafer. If, instead of tuning the filter to the wavelength of the mainlongitudinal mode, one tunes the laser to the notch wavelength of thefilter, then it is possible to introduce distortion in the outputspecturm of the laser. Hence, the characterization of mode partitioningwill not be accurate.

For most semiconductor laser applications, it is desirable to utilize anotch filter which is sufficiently narrow to reject only the main modewhile passing substantially all the side modes. Of course, the rejectionstrength must be sufficiently large so that one can view side modeactivity.

With the above characteristics in mind, it is possible to conceiveadditional optical notch filters which operate in accordance with thespirit and scope of the present invention. For example, it is possibleto use a dispersive element such as a grating or prism in combinationwith a mask to remove undesired wavelengths of light corresponding tothe main mode while recollecting the remaining light at the side modewavelengths. Of course, physical masking may be replaced by a techniquewhich involves a slotted reflector which recollects side mode light andpasses main mode light through the slot.

Additional wavelength selective optical filter implementations have beenrealized in fiber for wavelength division multiplexed communicationapplications. These implementations include a wavelength-selectiveinline bimodal-fiber tap described in both IEEE Photonics TechnologyLett., Vol. 2, No. 7, pp.484-6 (1990) and Proceedings of Conf. Opt.Fiber Commun., OFC '90, Paper WM2, p. 99, Jan. 22-26, 1990; aMach-Zehnder interferometer structure employing distributed Braggreflection gratings in each arm of the interferometer described inElect. Lett., Vol 23, pp. 668-9 (1987); and a helical-gratingbimodalfiber spatial mode coupler described in commonly assigned U.S.Pat. application Ser. No. 435,849 filed Nov. 13, 1989 and identified asC. D. Poole Case 1. The latter arrangement has been demonstrated inoptical fiber by coupling the light from a single mode fiber (the inputport) completely into only one of the modes of a length of bimodalfiber. Along this length of bimodal fiber, perturbations are applied tomix the light in the two modes. By introducing these perturbationsperiodically the light from only a narrow range of wavelengths willappear in one mode and all other wavelengths in the other. By couplingthe desired mode back into a single mode fiber the transfer function ofa notch filter is obtained. All the filter implementations in fiberlisted above lack tunability, narrow notch bandwidth, deep extinctionand polarization insensitivity.

While the Fabry-Perot etalon described above employs a standard aircavity, it is understood by those skilled in the art that solid orfilled cavity Fabry-Perot etalons and even fiber Fabry-Perot etalons maybe employed in the practice of this invention.

We claim:
 1. A method for observing mode partitioning activity in aradiated beam from a laser emitting said beam in a single mainlongitudinal mode and a plurality of side longitudinal modes, the methodcomprising the steps of:tuning an optical filter to a first wavelengthcorresponding substantially to said main longitudinal mode; separatingin said optical filter said main longitudinal mode of said radiated beamfrom said plurality of side longitudinal modes to provide substantiallyall of said plurality of side longitudinal modes in a predeterminedrange of wavelengths simultaneously at a first output of said opticalfilter, and detecting in a square law means a power characteristic ofsaid plurality of side longitudinal modes output at the first output ofsaid optical filter.
 2. The method as defined in claim 1 furtherincluding the steps of:outputting said power characteristic from saidsquare law means, comparing said power characteristic output from saidsquare law means to a predetermined threshold, and indicating occurrenceof a mode partitioning event when said power characteristic exceeds saidthreshold.
 3. The method as defined in claim 1 further including thestep of accessing via a second output of the optical filter said mainlongitudinal mode.
 4. A method for observing mode partitioning activityin a radiated beam from a laser emitting said beam in a single mainlongitudinal mode and a plurality of side longitudinal modes, the methodcomprising the steps of:tuning an optical filter to a first wavelengthcorresponding substantially to said main longitudinal mode by spatiallyseparating a first and second reflector elements, which form aFabry-Perot etalon, a predetermined distance from each other to causetransmission of the first optical wavelength therethrough whilemaintaining both reflector elements in a substantially parallelrelationship, positioning the first reflector element to have its normalaxis tilted by an angle γ to a longitudinal axis of said radiated beam,positioning a third reflector element external to the Fabry-Perot etalonadjacent to, and in a substantially parallel relationship with, thefirst reflector element separated therefrom by a distance L to permitmulti-passing reflected component of said optical beam between saidFabry-Perot etalon and said third reflector, directing said radiatedbeam having a collimated beam width α onto the first reflector elementas an input port to the optical filter separating in said optical filtersaid main longitudinal mode of said radiated beam from said plurality ofside longitudinal modes to provide substantially all of said pluralityof side longitudinal modes in a predetermined range of wavelengthssimultaneously through a second output port at the first reflectorelement of said optical filter, outputting through a first output portat said second reflector element said single main longitudinal modesubstantially at said first optical wavelength, outputting through thesecond output port at said first reflector element said plurality ofside longitudinal modes in the range of predetermined wavelengths, saidsecond output port being spatially separated from said input port, anddetecting in a square law means a power characteristic of said pluralityof side longitudinal modes output at the first output of said opticalfilter.
 5. The method as defined in claim 4 furtherincluding,collimating said radiated beam being supplied to the inputport at the first reflector element, and focussing said optical beamoutput from the second output port of the first reflector element. 6.The method as defined in claim 4 further including the stepsof:outputting said power characteristic from said square law means,comparing said power characteristic output from said square law means toa predetermined threshold, and indicating occurrence of a modepartitioning event when said power characteristic exceeds saidthreshold.